The present invention relates to systems for distributing electrical power. More particularly, the present invention relates to methods and systems for monitoring, protecting and controlling an electrical power distribution network.
It is important to monitor, provide protection, and control systems which distribute electrical power, and many techniques have been used to provide these functions.
Referring now to FIG. 1, a conventional group of protective functions applied to a power transformer and two circuit breakers arranged in a breaker-and-a-half system configuration is shown. The protection scheme includes, current transformers 14, 16, and 18, and voltage transformer 20. It should be appreciated that FIG. 1 is a single line representation of a three-phase system. The scheme of FIG. 1 thus provides the following available alternating current (AC) values: three-phase currents (from each set of current transformers 14, 16, and 18), and three-phase voltage (from voltage transformer 20). FIG. 1 further includes indications of desired protection and metering, including 50BF (instantaneous overcurrent) breaker failure protection for the breakers at points 24 and 26, 87T (current differential) transformer protection across main transformer 22 (represented as 28), 50P (instantaneous phase overcurrent) transformer protection at point 30, and a measurement of Watts at point 32.
FIG. 2 shows a conventional relay application for achieving the desired protection and metering goals of FIG. 1. The relay application includes first and second 50BF protective relays 34 and 36, which receive inputs from current transformers 14 and 16, respectively, and which provide outputs to a summing means 38. The 50BF protective relays provide the desired breaker failure protection. There is also provided a multifunction transformer protection relay 40, which receives inputs from the external summing means 38, current transformer 18, and a voltage transformer 20. The external summing means 38 outputs a summation of the AC current values derived from the transformers 14 and 16.
The transformer differential relay 40 receives voltage data from the voltage transformer 20 via voltage sensor 42, receives summed current values from the external summing means 38 via current sensor 44, and receives current values from the current transformer 18 via current sensor 46. As shown in FIG. 2, the transformer differential relay 40 provides the desired power metering by processing the voltage received at voltage sensor 42 with the summed current values from the external summing means 38. Further, the transformer protection relay provides the desired phase instantaneous overcurrent protection based on the summed current values from the external summing means 38, and provides the desired transformer differential protection based on both the summed current values from the external summing means 38 via current sensor 44 as well as from the current transformer 18 via current sensor 46. The transformer protection relay 40 typically includes a single digital signal processor to perform necessary calculations and provide protective control functions. It should be noted that by summing power system data in an external summing means, the relay is unable to determine the individual components of the summed data values.
U.S. Pat. No. 5,224,011 discloses a multifunction protective relay system which implements a dual processing architecture, using a first digital signal processor (DSP) to execute signal processing algorithms, and using a separate digital signal processor for input/output data processing. A dual-ported random access memory (RAM) is used to allow the separate DSPs to communicate with each other. The protective relay selectively trips and closes a circuit breaker at a generator or cogenerator site, or at a site which connects it to an electric utility system.
U.S. Pat. No. 5,828,576 discloses a power monitoring apparatus and method with an object oriented structure. Individual monitoring devices are used, each of which receives an electrical signal and generates a digital signal representing the electrical signal. Objects within each device include functional modules and registers, which contain inputs, outputs, and setup information for the modules. The function and configuration of each individual monitoring device can be changed. At least one module within the device receives the digital signal as an input and uses the signal to generate measured parameters, and additional modules can generate further parameters from the measured parameters.
While it is typically desirable to perform metering in a power distribution system, conventional protective relays do not adequately perform this function. The dynamic range requirement for protection at 0 to 20-50 times rated current input (typically 1A or 5A) results in a reduction of accuracy from the instrument transformer in the normal metering range at 0 to 1.5-2 times rated current input. The wide dynamic range also results in reduced accuracy and resolution for the measuring device sub-system (e.g., microprocessor, analog/digital converter, and associated analog conditioning circuitry). While certain protective relay devices can provide relatively accurate metering through current transformer inputs, in practice these inputs are typically connected to relaying class current transformers to ensure that the relay provides adequate protection. Relaying class current transformers typically have an accuracy of approximately 5-10%.
It follows from the above that it would be desirable to be able to derive power system data from many points on a power distribution system in a single relay, and it would be further desirable if the points from which the power system data could be derived were configurable by a user. Further, it would be desirable to be able to provide revenue-class metering in a protective relay to simplify installation and system integration, and to provide a protective relay user the ability to easily perform revenue class metering to verify the accuracy of utility company charges. Conventional protective relays do not adequately provide these capabilities.
The present invention overcomes the above-noted deficiencies, and achieves additional advantages, by providing for, in an exemplary embodiment, a protective relay device with multiple digital signal processors which receive configuration commands via a user interface. The configuration commands define source points in a power distribution system from which electrical parameters can be derived. The derived data can be combined within the relay, allowing for a wide variety of measured parameters and protection capabilities within a single device. Further, the use of multiple digital signal processors allows the protective relay to perform revenue class metering, in addition to the protection functions, by implementing a different dynamic range.
According to an exemplary method of the present invention, data can be derived from a power distribution system by the steps of: configuring a protective relay, through a protective relay interface, to receive system data from a plurality of source points; sensing system parameters at the source points; and performing network monitoring and control in the protective relay based on the sensed system parameters. The data can be combined within the protective relay to provide a wide variety of protective control options. Further, the dynamic measurement range for a given source point may be modified, allowing the protective relay to perform revenue-class metering from one source in addition to protective functions from another source.